We have studied the reactions occurring in the solid state in phenol-formaldehyde systems crosslinked by a base. The results obtained with chemically defined models with Fourier transform infra-red spectroscopy and solid-state C-13 nuclear magnetic resonance spectroscopy (cross-polarization/magic angle spinning) applied the study of different resoles with variable formaldehyde/phenol ratios have shown that changes in the prepolymer can be followed after the gel point. Nevertheless, data obtained on the final structure of the system indicate that the fine characterization of the prepolymer before the gel point must be conducted in the study of these complex systems.